A receiver capable of receiving TV and FM-radio signals is described in U.S. Pat. No. 5,148,280. The prior-art receiver comprises a single tuner for both TV and FM-radio reception. The single tuner frequency converts both a selected FM-radio signal and a selected TV signal to a fixed intermediate frequency (IF) signal of approximately 40 MHz. An IF of approximately 40 MHz is common for TV reception.
The IF signal produced by the tuner is processed differently for TV or FM-radio reception. For TV reception, the IF signal is processed in a manner which is very similar to IF signal processing in most present-day TV receivers. The IF signal is applied to a TV IF demodulation unit via a surface acoustic wave (SAW) filter. The TV IF demodulation unit provides a base-band composite video signal and a TV sound carrier signal. The TV sound carrier signal is further processed in a TV sound channel, which provides, in response, base-band audio signals.
For FM reception, the IF signal is supplied to a single chip FM-radio integrated circuit (IC) via a filter unit which includes a 43.3 MHz band-pass filter and a 48.65 MHz trap. In the FM-radio IC, the IF signal is frequency-converted to obtain a nominal 10.7 MHz FM IF signal. The FM IF signal is filtered by a ceramic filter arrangement and amplified, detected and decoded in the FM-radio IC. Thus, for FM reception, the prior-art receiver employs a double conversion scheme. The tuner performs a first frequency conversion and the FM-radio IC performs a second frequency conversion.
The filter unit, which is coupled between the tuner and the FM-radio IC, has to satisfy relatively strict requirements in order to avoid excessive distortion of the FM-radio signal to be received. First, the pass-band of the filter unit needs to be sufficiently close to 43.3 MHz. This requirement follows, among other things, from the frequency characteristic of the tuner, which is haystack-like, and the characteristics of the signals to be received. Secondly, the 46.85 MHz trap included in the filter unit needs to be accurately tuned to that frequency so as to avoid an image reception problem in the second frequency conversion.
Although the receiver described in U.S. Pat. No. 5,148,280 uses a single tuner for both TV and FM-radio reception, implementations of the prior-art receiver will be relatively spacious. The filter unit, which is coupled between the tuner and the FM-radio IC, is built up with discrete capacitors and inductors. In order that the filter unit satisfies the above-described requirements, the inductors need to have a certain size. If not, the inductors will have a too low quality factor to achieve an adequate selectivity and accuracy.
A more recent state of the art ‘FM+TV radio in one concept’ input section, similar to as the one described in U.S. Pat. No. 6,125,269 has typically two switching elements, apart from a possible antenna input switching using switching diodes. First, an oscillator is switched to a lower IF (10.7 MHz), and second, a band-pass-filter is switched from a TV band-pass of ˜10 MHz bandwidth to a lower 2 MHz bandwidth. The input-signal itself however is not switched. Selectivity and image rejection is achieved in the band-pass filter and augmented by two traps at the input for two specific frequencies in order to, e.g., fulfill CENELEC. The bandwidth of an FM channel is however only 200 kHz compared with a TV channel in VHF of either 6,7 or 8 MHz.
FIG. 1 illustrates a block-diagram of a current state of the art combined FM-input and TV VHF low band input.
FIG. 2 illustrates an equivalent circuit for the FM-radio input and TV-VHF-low-input, however eliminating switching diodes (switch 130 in FIG. 1) that are necessary for isolating between TV VHF-low input and FM-radio input. The two traps behind the FM-antenna input terminal (tuned at 119 MHz and 130 MHz) attempt to provide sufficient image rejection for the FM radio channels. The Mosfet amplifier 150 and the tuned filters 160 and 170 behind the Mosfet amplifier of FIG. 1 are not shown in FIG. 2.
FIG. 3 shows a typical response of a TV VHF-low band filter for the FM antenna.
An FM-concept in an RF tuner capable of receiving TV and FM-radio signals of the prior art have a poor selectivity due to band-with of mentioned filters and a used damping resistor. The poor selectivity can in turn cause oscillator radiation at the FM-antenna input terminal. The FM-input traps provide only a poor image rejection.